USDA Forest Service photo by Justin LaMountain. Danielle Kelley, iUSFS State, Private, and Tribal Forestry staff, releases predatory beetles as a biocontrol for hemlock woolly adelgid in Allegheny National Forest.

Hungry, hungry beetles continue to be the U.S. Agriculture Department’s best bet to combat the hemlock woolly adelgid, a non-native invasive species that poses a threat to the forest.

Hemlock woolly adelgid is an invasive insect native to Japan and the Pacific Northwest. The adelgid poses a serious threat to eastern and Carolina hemlock, which are at risk of fatal infestations since they lack natural resistance to the hemlock woolly adelgid. Within its native areas the hemlock woolly adelgid is kept in check by natural predators, while hemlock trees have adapted to the insect. Forests in the eastern part of the United States don’t have natural predators for the hemlock woolly adelgid and native hemlock trees haven’t adapted to the pest.

During October and November more than 2,000 predatory beetles were released at several sites across the Bradford Ranger District of Allegheny National Forest. In early October, roughly 1,000 L. nigrinus beetles were released at two sites near Yellow Hammer and Muzette. Then, in early November, roughly 1,000 L. osakensis beetles were released at two sites near Hector Falls.

Previous hemlock woolly adelgid biocontrols were used in the Allegheny National Forest as part of the Integrated Pest Management and Hemlock Conservation strategy for the Allegheny Plateau.

Since 2020, 5,600 predatory beetles and 7,300 predatory flies have been released at more than a dozen sites across the forest. All flies and beetles were provided by Cornell University and Virginia Tech, with technical support from US Forest Service State, Private, and Tribal Forestry.

Biological control is the use of a natural predator to manage pest populations. Four insect species are known to be effective biocontrols for hemlock woolly adelgid: predatory beetles (Laricobius nigrinus and L. osakensis) and silver flies (Leucotaraxis argenticollis and L. piniperda). The species are native to the Pacific Northwest and Japan, where hemlock wooly adelgid is a common pest of local hemlocks.

Laricobius beetles are released as adults in the fall and feed exclusively on developing and adult HWA throughout the fall and winter while the adelgid is in the first of two annual generations. Leucotaraxis flies are released as adults in the spring, with larvae preying on HWA eggs throughout the adelgid’s laying season. When present on the same site in sufficient numbers, the two biocontrols work in tandem for increased predation across multiple HWA life cycle stages.

Adelgids are a small family of insects closely related to aphids and feed on plant sap. Feeding from adelgids interferes with the tree’s use of nutrients, and in the case of hemlock, causes needle drop, branches to die and eventually trees to die. The egg sacs of these insects look like the tips of cotton swabs clinging to the undersides of the current year’s hemlock twigs.

The hemlock woolly adelgid was accidentally introduced to Virginia from Japan in the 1950s, and by the late 1960s was reported in southeastern Pennsylvania. By the end of 2023, the pest was found in all 67 Pennsylvania counties.​